Recently, in the field of radiography for dental treatment, there has been a transition from conventional photography using X-ray sensitive films to imaging by image pickup devices using camera tubes such as X-ray image intensifiers that do not use films, solid-state image pickup devices for which scintillates and CCDs are combined, and the like (see Patent Document 1, for example). However, the image intensifiers are camera tubes, and thus have problems such that the size is large, and an image distortion occurs in the periphery of an imaging region. Moreover, the CCDs, since it is technically difficult to form imaging regions with large areas, have restrictions such as performing imaging by a line scanner system.
As an image pickup device for solving these problems, a CMOS-X-ray flat panel sensor can be mentioned. The X-ray flat panel sensor uses a CMOS solid-state image pickup device having an imaging region in which a plurality of pixels respectively including photodiodes are arranged two dimensionally, and thus has no distortion in a peripheral portion, and it is easy to provide the imaging region with a large two-dimensional area (for example, about 12 cm×12 cm). Unlike visible light, since X-rays cannot be condensed by a lens, the solid-state image pickup device requires an imaging region with a large area when imaging a large area at one time, such as when being applied to dental use (particularly, dental 3D-CT).
Moreover, for such a solid-state image pickup device for medical use, the imaging subject is, for example, a living body, and thus the required resolution is low, and moreover, it is necessary to improve sensitivity while suppressing the amount of X-ray exposure as much as possible. Based thereon, in the solid-state image pickup device for medical use, the area of a photosensitive region (region to generate an electric charge in response to incidence of light) of each pixel included in the imaging region is no less than about 10 times to 100 times larger than that of another solid-state image pickup device for general imaging.
In, for example, a solid-state image pickup device for dental 3D-CT, VOXELs to be used for reconstruction of a CT image have square shapes, and it thus becomes indispensable that the shape of a photosensitive region of each pixel is a square one side of which has a length of, for example, 150 μm to 200 μm. Since the photosensitive region of each pixel thus having a large area means that the number of pixels included in the imaging region is small, this becomes advantageous in respect to performing drive at a video rate of 30 frames/second.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2005-333250